1. Field of the Invention
The present invention relates generally to electronic current and voltage references.
2. Description of the Related Art
FIG. 1 shows a conventional current source 20 which is especially suited for use in complementary metal-oxide semiconductor (CMOS) integrated circuits. The reference 20 has an n-channel metal-oxide field-effect transistor (NMOSFET) that is arranged as a "diode-connected" transistor 22. The drain of this transistor is coupled to a supply voltage V.sub.DD through a drain resistor 24 and its gate is coupled to the gate of an output NMOSFET 26. The sources of both transistors are connected to ground and and the drain of the transistor 26 serves as a current port 28.
In operation of the current source 20, a reference current 30 is set by the expression {V.sub.DD -V.sub.DS22(sat) }/R.sub.24 in which V.sub.DS22(sat) is the saturation voltage of transistor 22 and R.sub.24 is the resistance of the drain resistor 24. Because of the circuit structure, transistors 22 and 26 have the same gate-to-source voltage V.sub.GS. If they have the same physical layout and are proximate to each other in an integrated circuit, they have substantially the same operating characteristics so that their identical V.sub.GS voltages cause an output current 32 to be substantially equal to the reference current 30 (the output transistor 26 has a finite output resistance r.sub.o so that the output current 32 will vary somewhat with the drain-to-source voltage V.sub.DS across this transistor).
The reference current 30 appears to be reflected in the output and, accordingly, the current source 20 is typically referred to as a "current mirror". The output current 32 of the current mirror 20 is substantially proportional to the supply voltage V.sub.DD and is generally sensitive to temperature.
Virtually all references are based on a voltage standard and reduced sensitivity to supply voltage has generally been obtained by replacing it with a different standard. For example, the conventional biasing source 40 of FIG. 2 is typically referred to as a V.sub.t -referenced source because its voltage standard is the MOSFET threshold voltage V.sub.t (threshold voltage being that V.sub.GS voltage that initiates channel inversion in a MOSFET).
In the current source 40, a resistor 42 is coupled between the gate and source of an NMOSFET 44. The transistor 44 is coupled to V.sub.DD through a PMOSFET 46 and the resistor 42 is coupled to V.sub.DD through an NMOSFET 48 and a diode-connected PMOSFET 49. The gate of transistor 48 is connected to the drains of transistors 44 and 46 and transistors 46 and 49 are gate-coupled. An output PMOSFET 50 is gate-coupled to transistors 46 and 49. The drain of transistor 50 forms a current port 52 and the source of the transistor 48 forms a voltage port 54.
In operation of the reference 40, transistors 46 and 49 form a first current mirror 60 that is similar to the current mirror of FIG. 1 and transistors 49 and 50 form a second current mirror 62. Because of the current mirror 60, the reference current 64 and the current 66 through transistors 44 and 48 are substantially equal. In addition, the reference current 66 through resistor 42 sets the V.sub.GS voltage of transistor 44. Combining this relationship with a well-known expression for V.sub.GS (e.g., see Gray, Paul R., et al., Analysis and Design of Analog Integrated Circuits, John Wiley and Son, third edition, 1993, New York, p. 64) yields ##EQU1## in which I represents the currents 64 and 66, R.sub.42 is the resistance of resistor 42, .mu. is the channel carrier mobility of transistor 44, C.sub.ox is gate oxide capacitance per unit area of transistor 44 and W/L is the channel width-to-length ratio of transistor 44.
If the ratio W/L is large, the root term in equation (1) can be neglected which leaves V.sub.GS equal to V.sub.t. Because the voltage across the resistor 42 is then substantially V.sub.t, the reference current 66 is approximately V.sub.t /R.sub.42 and the current mirrors 60 and 62 force the current 64 and an output current 70 to also approximate V.sub.t /R.sub.42. Because the voltage at the voltage port 54 is that across the resistor 42, it is substantially V.sub.t.
The current source 40 is arranged to be a self-biasing structure and such structures typically exhibit an undesired zero-current operating point in addition to the intended operating point. The current source 60 forces the currents 64 and 66 to be equal while the connection of resistor 42 to the gate of transistor 44 forces the gate-to-source voltage V.sub.GS of transistor 44 to equal the current-induced voltage across the resistor.
There are two places where both of these currents and voltages are equal. One is the intended operating point described above and the other is at a zero-current state. If some current initially flows in the current source 40, it will drive itself to the intended operating point. To insure that the current source 40 is driven to this stable operating point, therefore, a startup circuit 72 is arranged to inject a starting current into the structure of the reference 40.
As shown above, the current source 40 provides a current 70 that approximates V.sub.t /R.sub.42. Therefore, in contrast to the current 32 of the current source 20 of FIG. 1, this current is substantially independent of supply voltage. Unfortunately, the threshold voltage V.sub.t is temperature sensitive and the resistance of the resistor 42 is typically temperature sensitive so that the output current 70 changes over temperature.
In order to remove this temperature sensitivity, other conventional references are arranged to oppose the negative temperature coefficient of the threshold voltage V.sub.t with the positive temperature coefficient of the base-emitter voltage V.sub.BE of a bipolar transistor. Because V.sub.BE exhibits a greater temperature coefficient, these reference circuits typically multiply V.sub.t by a constant K before summing it with V.sub.BE to generate a temperature insensitive voltage reference V.sub.R.
In a semiconductor transistor, the base-emitter voltage is a function of the semiconductor's band-gap voltage V.sub.GO at zero degrees Kelvin and, as a consequence, an expression for V.sub.R generally includes the band-gap voltage term V.sub.GO. Accordingly, these references are typically referred to as "band-gap references".
Although band-gap references can be essentially temperature insensitive, they are generally sensitive to variations in fabrication processes. If the circuits of an integrated circuit respond in a similar manner to process variations, this correlation can be used to reduce the the integrated circuit's process sensitivity. This reduction is difficult to realize with band-gap references because the process sensitivity of V.sub.BE lacks the necessary correlation to the process sensitivity of CMOS circuits.
Conventional references are therefore generally sensitive to at least one of the parameters of supply voltage, temperature and fabrication processes.